The present disclosure is related to orthopedic devices implanted between skeletal segments. The implanted devices are used to adjust and maintain the spatial relationship(s) of adjacent bones. Depending on the implant design, the motion between the skeletal segments may be returned to normal, increased, modified, limited or completely immobilized.
Whether from degenerative disease, traumatic disruption, infection or neoplastic invasion, alterations in the anatomical relationships between the spinal vertebras can cause significant pain, deformity and disability. Spinal disease is a major health problem in the industrialized world and the surgical treatment of spinal pathology is an evolving discipline. The traditional surgical treatment of abnormal vertebral alignment and aberrant motion is the complete immobilization and bony fusion of the involved spinal segment. More recently, preservation of vertebral motion during the treatment of the spinal pathology has been the preferred strategy and many surgical techniques have been formulated to accomplish this treatment objective.
Regardless of whether the vertebral motion is abolished or preserved, many surgeons employ implantable orthopedic devices that adjust, align, support and/or maintain the spatial relationship(s) of the adjacent vertebral bones. The effectiveness of theses devices is vitally dependant on the adequacy of their fixation onto the underlying bone. Inadequate device fixation will effectively uncouple the device from the vertebral column and marginalize the beneficiary effects of the implant. Further, poorly anchored devices may damage the attached bone by fracturing and/or avulsing bone fragments at the attachment sites.
Screw fixation into the pedicle portion of the vertebral body has emerged as the most common method of device fixation onto the vertebral column. However, it is known that repeated loading and unloading of these screws will lead to screw loosening and eventual pull-out. Implantable devices that promote spinal fusion must bear load for the few months needed to produce bone graft maturation and solid vertebral fusion. In contrast, devices that preserve vertebral motion must bear the cyclical load of movement for the remainder of the patient's life. With the change in treatment strategy towards motion preservation, the integrity of the bone/device interface and the durability of the device fixation sites are emerging as major determinants of implant's functional life span.